July 11, 2013

Glass Sponge Community Thrives Despite Antarctic Ice Loss

The waters around Antarctica can be an inhospitable place, but a new study has shown that dynamic events in the region are constantly shaping and even boosting the ecosystems in the Southern Ocean.

The study, published in Current Biology, reports on a surprisingly prolific community of glass sponges colonizing an area formerly covered by permanent ice - surprising because the sponges live long, slow lives that can extend up to 10,000 years.

"By comparing identical tracks video-surveyed by remotely operated underwater vehicle in one of the least accessible parts of the Antarctic, we found two- and three-fold increases in the biomass and abundance of glass sponges, respectively, from 2007 to 2011," said co-author Claudio Richter of the Alfred Wegener Institute in Germany. "This is much faster than any of us would have thought possible."

In 1995, the Larsen A ice shelf collapsed into the western Weddell Sea. Scientists had assumed that affected ecosystems there would slowly change over time due to the very low temperature and intermittent resources in the icy waters.

"We now know that glass sponges may undergo boom-and-bust cycles, allowing them to quickly colonize new habitats in a short period of time," Richter said.

"To the organisms living on the sea bed, the disappearance of the hundred-[meter]-thick Larsen A ice shelf must have been like the heavens opening up above them," he added.

The ice shelf collapse had alleviated the darkness that once dominated the area, allowing sunlight to facilitate plankton growth. The exploding plankton population created a steady food supply for glass sponges, which feed on the smallest plankton by filtering it from the water. As the sponges grow, their vase-like bodies provide spawning and sheltering opportunities for fish and other Antarctic sea inhabitants.

"Like corals, sponges create their own habitats," Ricther said. "To an extent they are like cities on the sea bed. There is something going on wherever they grow, and this attracts other sea dwellers to them."

While the ecosystems appeared to benefit from the ice shelf collapse, the researchers said it would be premature to draw the conclusion that loss of ice induced by climate change would be beneficial for the Antarctic.

"There are still too many unknowns to make predictions," said co-author Laura Fillinger, a biologist at the Alfred Wegener Institute. "One example is the question of the influence of competitors: currently we are witnessing a fierce competition for space on the sea bed."

"Another concerns predators: in our dive in 2011 we hardly saw any of the snails and starfish, which feed on glass sponges," she added. "However, it is possible that these voracious predators will follow suit and wreak havoc."

"A general principle to be learned from our study is that benthic communities are very dynamic, even under the extreme environmental conditions prevailing in the Antarctic," Fillinger said. "Only four years ago, the study area was dominated by a species of sea squirt. Now this pioneer species has all but disappeared, giving way to a community dominated by young individuals of a glass sponge."